Social enviroment influences aphid production of alarm pheromone

In most aphid species, the volatile sesquiterpene (E)-beta-farnesene (E beta f) is released as an alarm pheromone in response to predation and is also emitted continuously at low levels. Some aphid predators use E beta f as a foraging cue, suggesting that the benefits to aphids of signaling via E beta f must be weighed against the cost of increasing apparency to natural enemies. To determine whether aphids vary E beta f production in response to features of their social environment, we compared the production of E beta f by Acyrthosiphon pisum (Harris) individuals reared in isolation with that of individuals reared among conspecifics or individuals of a different aphid species, Myzus persicae. Production of E beta f by A. pisum reared in isolation was significantly lower than that of aphids reared among conspecifics or among M. persicae individuals. When we reared A. pisum individuals in isolation but exposed them to odors from an aphid colony, E beta f production was similar to that of aphids reared among conspecifics, suggesting that aphids use a volatile cue to assess their social environment and regulate their production of alarm pheromone. It is likely that this cue is E beta f itself, the only volatile compound previously found in headspace collections of A. pisum colonies. Finally, we examined the attraction of a predatory hoverfly, which uses E beta f as a foraging cue, to groups of aphids reared in isolation or among conspecifics and found that groups comprising individuals reared in isolation were significantly less attractive to the predator, suggesting that the observed variation in E beta f production may be ecologically relevant.